In manufacturing semiconductor devices such as LSI and super-LSI or in manufacturing a liquid crystal display panel or the like, a circuit pattern is made by irradiating a ultraviolet light to a semiconductor wafer or a glass plate for liquid crystal, but if a dust particle is sticking to a photomask used in this stage, the dust particle obstructs or reflects the ultraviolet light, causing deformation, short circuit or the like in the pattern being transferred, and such phenomena would lead to lowering of the quality of the end products.
Thus, these works are usually performed in a clean room, but, even in a clean room, it is yet difficult to keep the photomask clean all the time. Hence, the exposure light irradiation is conducted only after a pattern-including part of the surface of the photomask is sheltered by a pellicle as a dust fender. Under such circumstances, foreign particles do not directly adhere to the surface of the photomask, but only onto the pellicle membrane, and thus by setting a photo focus at a pattern on the photomask at the time of lithographing, the foreign particles on the pellicle membrane do not affect the transferred image.
In general, a pellicle is built up of a pellicle frame, which is made of an aluminum alloy, a stainless steel, an engineering plastic or the like, and a transparent pellicle membrane made of cellulose nitrate, cellulose acetate, a fluorine-containing polymer or the like which transmit light well; this pellicle membrane is adhered to an upper one of the two annular faces of the pellicle frame. On a lower one of the annular faces of the frame is laid an agglutinant layer made of a polybutene resin, a polyvinyl acetate resin, an acrylic resin, a silicone resin or the like for attaching the pellicle frame to the photomask, and over this agglutinant layer is laid, if need be, a releasable liner (separator) for protecting the agglutinant layer.
In recent years, owing to the increased refinement of the exposure light pattern, the problem of the deformation of the photomask caused by pellicle adhesion to it has become more focused. This problem occurs as the photomask and the pellicle are coupled together via the agglutinant layer, whereby the form of the pellicle frame affects that of the photomask, and thus the pattern described on the surface of the photomask is deformed from the original form; consequently, a kind of pellicle which induces very little change in the shape of the photomask as it is adhered to the photomask is called for.
Various countermeasures have been proposed to solve this problem: for example, the mask-boding agglutinant layer is made softer, or the flatness of the pellicle frame is improved. These proposed methods can reduce the effect the pellicle frame shape imparts to the photomask shape, but the result is not necessarily sufficient. This is on account of the fact that the flatness of the pellicle frame and that of the photomask are not perfect and, depending on the combinations of them the magnitude of the effect imparted by the pellicle frame shape to the photomask shape is diversified.
It is theoretically preferable that a pellicle frame is easy to deform so as to readily imitate the shape of the photomask, which is enabled through reduction of the rigidity of the pellicle frame as much as permissible; and proposals have been made as to how to do this, such as a use of a material of low rigidity like resin, or lowering the height of the pellicle frame, or modifying the cross section of the frame's side bar to reduce its cross-sectional area whereby the rigidity of the frame is reduced. (ref. IP Publications 1, 2 and 3)
On the other hand, in most exposure apparatuses, the photomask is held horizontally during the exposure stage, so that there has been a problem of deflection of the photomask induced by its own weight during this stage. When an excessive deflection occurs in the photomask, the pattern for exposure described on its surface incurs deformation, and cannot effect exposure of the pattern as designed. In order to solve this problem, methods are proposed such as: as a normal method the thickness of the photomask is increased in relation to the size of the photomask to thereby restrict the amount of the self-weight deflection to a predetermined value (ref. IP Publication 4); or in cases of large-sized exposure apparatuses the deflection of the photomask is corrected by means of application of a negative pressure or the like (ref. IP Publication 5); or anticipating the effect of the self-weight deflection of the photomask, it is cancelled in the optical system (ref. IP Publication 6).
Based on an assumption that the photomask is normally made perfectly flat, the pellicle or pellicle frame is also made as flat as possible, so that the higher the flatness of the pellicle frame is, the better it is considered; however the fact is that during use (during exposure) the flatness of the photomask (which degrades due to deflection) and that of the pellicle are not necessarily the same. Hence, in order to bring the flatness of each of these closer to each other even during the exposure, a consideration is made to increase the plate thickness of the photomask further so as to minimize the self-weight deflection; but this measure has a problem of pushing up the cost and the weight of the photomask since it is made of costly and heavy synthetic quartz.
As a different measure, it is possible to substantially reduce the rigidity of the pellicle frame so as to allow the pellicle frame to assimilate itself to the shape of the photomask. However, preferably the rigidity of the pellicle frame is as high as possible, so as to maintain tenseness of the pellicle membrane it bears, and also to prevent the pellicle frame from undergoing deformation during stages such as its manufacturing and its adhesion to the photomask, which causes the pellicle membrane to have wrinkles; therefore, a measure of substantially reducing the rigidity of the pellicle frame faces problems in manufacturing and handling. For this reason, this measure has a limit in that the rigidity of the pellicle frame cannot be reduced very much.
As explained above, during the exposure operation, the photomask has a deflection caused by the weight of itself; however, in fact the shape of the photomask coupled with the pellicle during the exposure operation is a complex result of two elements: one the deformation of the photomask purely caused by its self-weight, and the other the deformation of the photomask purely induced by the shape of the pellicle, and these two shapes resisting each other bring about the resulting deflection or the shape of the photomask; and it is noted that no pellicle frame or pellicle have been designed based on a consideration of this complex effect that the pellicle will give to the photomask during the exposure operation.